> What I can't see is rationale in inventing yet another language
I can see that, if it's a significant improvement on the previous language for the user base.
That's not the case for Dart, I see a lot of truth in the quips "puts the Java back into Javascript" and "Takes a hard turn towards going back to more machine-friendly and less developer-friendly languages".
New languages are a fine thing, but Dart is neither original nor interesting, it's mostly shoving back a heap-ful of Java into Javascript and calling it a day, that's not helpful.
Some sort of web IR all web languages could compile to would be useful (especially with broad support from other browser vendors), a chopped down Javascript with its problems fixed (JS 1.7's `let` instead of current `var`, "strict-mode" by default, structural types, typed object keys, an actual integer type) would be helpful, a "best subset" of Harmony would be helpful, Harmony itself would be helpful, a brand new language with PL concepts less than 30 years old would be helpful, but from what I've seen so far Dart is none of these.